(1) Field of the Invention
The present invention relates to the delivery of power to an intermittently energized load. More particularly, this invention is directed to power supplies for gaseous discharge tube devices. Accordingly, the general objects of the present invention are to provide novel and improved methods and apparatus of such character.
(2) Description of the Prior Art
While not limited thereto in its utility, the present invention has been found to be particularly well suited for controlling the operation of warning lights and particularly for use in warning light systems which employ xenon flash tubes. Such warning light systems are well known in the art and find application on emergency vehicles, aircraft and in other installations where it is considered necessary or desirable to attract attention by means of the generation of intermittent bursts of energy in the visible range of the frequency spectrum. For disclosure of prior art devices for controlling the energization of flash tubes, reference may be had to U.S. Pat. Nos. 3,515,973 and 4,013,921; both of these prior patents being assigned to the assignee of the present invention.
As discussed in U.S. Pat. No. 4,013,921, visibility of a warning light system may be enhanced by causing the lamp employed therein to be ionized twice in rapid succession. Electronic flash tubes, xenon tubes for example, produce a burst of light which is of comparatively short duration although the intensity of the light generated is extremely high. Thus, by causing the tube to flash twice in rapid succession, and thereafter have the customary dwell time which constitutes the major portion of the operational cycle, visibility will be enhanced since the net effect will either be a flash which appears to be of longer duration or a discernable double flash which aids in "fixing" the location of the light source.
Prior power supplies which have been suitable for use with flash tubes which are to be controlled to produce spaced groups of multiple flashes have been characterized by certain deficiencies. One of the more significant of these deficiencies resides in the fact that the prior power supplies were characterized by comparatively high power consumption and, incident thereto, the generation of a considerable amount of heat which had to be dissipated. This high power consumption and heat generation resulted from poor switching characteristics of the semiconductors through which current was delivered to a static inverter transformer primary winding, especially during "turn-off". High power consumption also resulted from an inability to disable the power supply (1) during the times that the flash tube was in the ionized state or (2) in response to the sensing of a current in excess of a predetermined safe amount or (3) in response to the sensing of an output voltage in excess of a preselected level.
To further discuss the poor switching characteristics of prior art power supplies, the current through the static inverter's power transformer was often controlled by a "power" transistor which was turned off by being current starved rather than being controlled so as to switch off cleanly and sharply. Operation in a current starvation mode resulted in an increase in the temperature to which the semiconductor device was subjected. Accordingly, it has previously been common practice to employ a pair of "power" transistors in parallel.
A further disadvantage of prior art power supplies suitable for use with intermittently energized loads resided in the fact that the active components, particularly the "power" transistor or transistors, had to be matched to the passive circuit components to insure against overdriving of the semiconductors. Thus, each power supply had to be tested in an effort to avoid drawing excessive current while insuring that the "power" transistors, when conductive, would operate in the region of saturation to minimize resistance and thus minimize heat generation.
A further disadvantage of prior art supplies designed for use with flash tubes resided in the inability to adjust the output of the power supply such that the intensity of the burst of light produced could be varies as a function of the ambient lighting conditions.